This invention relates generally to the field of closed loop, force balanced, inertial accelerometers. More specifically, it relates to a solid state (i.e., silicon), electrostatically-rebalanced accelerometer having electrostatic position pick-off of the sensing mass, and pulse-on-demand servo control.
Force balance sensing instruments, such as inertial accelerometers, often use a sensing member ("sensing mass" or "proof mass") that is movable from a nominal position in response to an input condition (e.g., inertial acceleration) that is to be sensed. A position pickoff provides a signal that is indicative of sensing member position, while a feedback signal, based on the pickoff signal, applies a force to the sensing member that tends to return it to the nominal position. The feedback signal may also provide an instrument output signal representing the sensed input condition. It is generally desired that the instrument output signal be proportional to the input condition. Thus, in many types of electrostatic and electromagnetic force balance sensing instruments, in which the forces applied to restore the sensing member to its nominal position are not linearly related to the feedback voltage or current supplied to the forcing means, special techniques are employed to obtain a linear relationship between the instrument output and the sensed input. Such linearization techniques are also dictated by the need to optimize the operation of the instrument itself by providing a feedback force applied by the feedback control network that has a linear relationship to the sensed input.
For example, in an electrostatic force balanced accelerometer, of the type disclosed in U.S. Pat. No. 4,679,434 to Stewart, electrostatic forcing in a closed loop system is employed to position and obtain an output signal from a pendulous inertial sensing mass. The electrostatic forcing system employs a capacitive pickoff electrode on each side of the sensing mass. The electrodes also apply nominally equal and opposite bias forces to the sensing mass, to which is applied a control voltage. In another control arrangement for an accelerometer of this type, a fixed bias voltage V is applied to the sensing mass, and feedback voltages +v and -v are applied concurrently to pickoff and forcing electrodes on opposite sides of the mass. Accordingly (omitting factors such as gap variation, parallelism, dielectric constants and the like, which may also affect the electrostatic forces), the force applied by each electrode to the sensing mass is proportional to (V+v).sup.2 and (V-v).sup.2, respectively. The net force applied to the mass by this control system is therefore the difference between these two forces, which is effectively proportional to 4 vV. As the bias voltage V is a constant, the feedback voltage, of magnitude v, is proportional to the feedback force applied, and it is also linearly related to the input acceleration experienced by the sensing mass.
The above-described system has a number of problems, including the large negative spring effect associated with the required bias electric fields. Even in the absence of any input acceleration to be sensed, the bias fields are required, and, since both the bias fields and the pickoff null position may vary, the instrument may have poor null stability and repeatability. In addition, such factors as gap variation, component aging, temperature variations, and the like, may provide sources of error that can result in spurious outputs and decreased null stability. Furthermore, small variations in electric field strength are exacerbated by the negative spring effect in voltage biased systems, caused by the two large bias fields, which effect may be unacceptably large for typical ranges of accelerometer inputs.
U.S. Pat. No. 5,142,921 to Stewart et al. discloses a force balanced instrument system in which the position of the sensing mass is capacitively sensed, and the mass is electrostatically forced toward a null position. A pickoff signal indicative of the positional displacement of the sensing mass is generated. Constant magnitude attractive forces are alternately applied to opposite sides of the sensing mass, with the duration of force application depending upon the position of the sensing mass and the resultant pickoff signal value. While this system addresses the linearity demands described above, its performance depends upon the application of equal charges to capacitive plates on either side of the sensing mass, which is sometimes difficult to achieve in practice. Moreover, because the system generates its pickoff signal by sensing voltages associated with the capacitive forcing charges, the sensing mass is subject to forces during the pickoff phase, which may be a source of error.
U.S. Pat. No. 5,277,053 to McLane et al. discloses an electrostatic, force balanced accelerometer system, in which a pickoff signal, indicative of the positional displacement of a sensing mass from a null position in response to inertial acceleration, is used to calculate a restoring force. A feedback signal, proportional to the square root of the restoring force, is applied to an electrostatic square law forcing circuit that applies an electrostatic restoring force to one side or the other of the sensing mass, by means of electrodes between which the sensing mass is pivotably mounted. A system output signal is proportional to the calculated restoring force, and thus is linearly proportional to the sensed acceleration. The transfer function of the forcing circuit is empirically determined by applying a series of known accelerations and measuring the balancing signal required to restore the sensing mass to its null position. The feedback signal is then generated, having a relationship to the pickoff signal that is the inverse of the empirically determined transfer function of the forcing circuit. A disadvantage of this system stems from its excitation of the electrodes by an AC signal to provide a high frequency pickoff signal that represents the sensing mass position. This results in a force being applied to the sensing mass during the position detection period, with possible resultant errors. In addition, the electronic circuitry required by this system is relatively complex and costly.
There has thus been a long-felt, but as yet unsatisfied need for a force balanced sensing system, particularly an accelerometer system, that eliminates or at least minimizes the shortcomings of the prior art. Specifically, such a system should minimize or eliminate such sources of error as negative spring effects and forces on the sensing mass during the position detection period. Such a system should also optimize null stability and repeatability, while minimizing spurious output signals. Finally, such a system should reduce the number, complexity, and costs of the components required to produce it, as compared with prior art systems.